Particle trajectory in a room environment is controlled dominantly by two forces, airflow, and electromagnetic fields. These two forces are the dominant transport mechanism for particles. Two equations dictate particle behavior. Force equals the change in momentum of the particle (F=ma), due to airflow. The airflow must overcome the charge times the electric field E (F=qE) due to electric forces in the room environment.
Note 1: F is the force, m is the mass, a is acceleration, and E is the electric field. Note 2: “E”, “F”, and “a” are vectors. This means the quantity has both magnitude and a direction. For example, E has both magnitude and direction.
The first equation (F=ma) describes how airflow controls particle trajectory and the second equation (F=qE) describes how the electric field controls particle trajectory.
When a media filter is placed in an airstream it has a pressure drop across it because it is placed perpendicular to the airflow. Air must pass through the media material. Pressure drop is the force required per unit of surface area that a fan must overcome to allow the proper airflow to pass through the filter material. The more efficient the filter, the more dense the material in the filter, and as a result the higher the pressure drop to allow the proper airflow through the filter. As an example, a HEPA filter can have over an inch and a half of static pressure drop across it.
Pressure drop is directly related to higher energy usage. The fan in an HVAC System must work harder to force air through the filter (FIG. 1). FIG. 1 illustrates that in order to maintain proper airflow across a high efficiency filter the fan in an HVAC air system must run at a higher rate which required more energy usage. Some Fans cannot operate under these high pressure drop conditions. The pressure drop across the system is ΔP=P1−P2. This means more energy usage which equates to more costs.
Some HVAC fans do not have the capability to operate under high pressure drop conditions. Furthermore, a fan that has the capability to create the acceptable pressure drop across a high efficiency filter must use more energy, in the form of kilowatt hours, and create more noise (unacceptable in certain environments, including hospital care facilities). These are the reasons it has been difficult to incorporate sufficient air purification in some of these HVAC systems. In any air handling system the struggle has always been to incorporate efficient filters and still maintain acceptable air flow rates through these systems. The result has been high energy costs to run the HVAC fan in the air conditioning system to provide the pressure drop needed to maintain acceptable airflow. Another example of a system that cannot withstand any pressure drop through it is the Chilled Beam Induction System, which is described in more detail below.
Therefore, in this disclosure a filtration system was developed with no, or very low, pressure drop across it. This system has acceptable filter efficiency without the associated pressure drop.
Aerosols are composed of either solid or liquid particles, whereas gases are molecules that are neither liquid nor solid and expand indefinitely to fill the surrounding space. Both types of contaminates exist at the micron and sub-micron level. Most dust particles, for example, are between 5-10 microns in size (a micron is approximately 1/25,400th of an inch). Other airborne contaminates can be much smaller. Bacteria and viruses are an example of airborne contaminates. Bacteria commonly range anywhere between 0.3 to 2 microns in size. Viruses can be as small as 0.02 microns in size. The importance of removing these contaminates varies based upon the application. Semiconductor clean rooms and hospital operating rooms are two examples of spaces where the ability to remove contaminates is critical. One factor complicating the removal of contaminates is that particle number density increases with smaller particle size. For example, in the typical cubic foot of outside air there are approximately 1000 10-30 micron sized particles. The same volume of air, however, contains well over one million 0.5 to 1.0 micron particles. Ultimately, over 98% of all airborne particles are less than a micron in size. The prevalence of small particles is problematic from an air quality standpoint because small particles are hard to control and capture. Transport Mechanisms are what causes particles in the air to move from point A to point B. In every building environment there are forces present that determine these transport mechanisms and control particle movement. The major types of forces on particles in a building environment are caused by airflow and/or electromagnetic fields (or forces). When a particle approaches a strong electrostatic field, say a negative 15 kV field, a dipole is formed. Some of the positive charges in the particle will move toward the strong field (front of the particle) and some of the negative charges will move towards the opposite end (rear) of the particle, away from the static field. Once this occurs the particle passes through the electrostatic field. If a second static field, of the same potential is downstream from the first static field the particle propels toward it. Attached to the second static field is a media material, made up of dielectric material (such as fiberglass) the particle propels into the media material and gets trapped. Thus the particle gets filtered, note FIG. 2. FIG. 2 illustrates that when a particle approaches the −10 kV electrostatic field it forms a dipole (A,B). If a second −10 kV electrostatic field is placed downstream from the first field the particle propels towards it (opposite charges attract) (C). If a dielectric media material is placed in the Second field it picks up the charge of the electric field and acts as a trap to the particle (D).
Electronic Charging of a Particle −A corona field is an ion field that is created by a very thin wire or a thin metal blade with a serrated edge. If a negative high voltage is applied to the wire or metal edge, electrons are created in the air surrounding the wire or blade. When a particle passes through this created electron field the particle acquires some of the electrons and becomes a negative ion. FIG. 3 illustrates this point. FIG. 3 illustrates that when a particle approaches the −15 kV electrostatic ion field it forms a negative ion out of the particle. If a second −15 kV electrostatic field is placed downstream from the first field the particle is deflected from it (like charges repel). If a +15 kV field is placed as above the negative ion is propelled toward it. As can be seen, when a particle passes through the negative ion field (electrons) it becomes negatively charged.
If a “V” shaped grid is placed in the path of the particle, and has the same voltage applied to it as the corona grid the particle will be repelled by it (like charges repel each other). If a positive set of grids are placed to the side of the first set of grids, as shown in FIG. 3, the particle will be propelled towards the positive grid (unlike charges attract each other).